Wind Farm (dfig Average Model)

Wind Farm (DFIG Average Model)

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power_wind_dfig_avg.mdl

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Wind Farm (DFIG Average Model) This demonstration illustrates simulation of a 9 MW wind farm using an average model of a Doubly-Fed Induction Generator (DFIG) driven by a wind turbine Richard Gagnon (Hydro-Quebec) Note: This demo uses a generic model of a DFIG wind turbine. The model is useful for education and academic works. Contents 1. Simulation Methods of the DFIG 2. Circuit Description 3. Demonstration

1. Simulation Methods of the DFIG Depending on the range of frequencies to be represented, three simulation methods are currently available in SimPowerSystems™ to model VSC based energy conversion systems connected on power grids. The detailed model (discrete) such as the one presentented in the “power_wind_dfig_det.mdl” model in the DR demo library. The detailed model includes detailed representation of power electronic IGBT converters. In order to achieve an acceptable accuracy with the 1620 Hz switching frequency used in this demo, the model must be discretized at a relatively small time step (5 microseconds). This model is well suited for observing harmonics and control system dynamic performance over relatively short periods of times (typically hundreds of milliseconds to one second). The average model (discrete) such as the one presentented in this demo. In this type of model the IGBT Voltage-sourced converters (VSC) are represented by equivalent voltage sources generating the AC voltage averaged over one cycle of the switching frequency. This model does not represent harmonics, but the dynamics resulting from control system and power system interaction is preserved. This model allows using much larger time steps (typically 50 microseconds), thus allowing simulations of several seconds.

1/9/2004 4:05 AM

Wind Farm (DFIG Average Model)

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The phasor model (continuous) such as the one presentented in the “power_wind_dfig” model in the DR demo library. This model is better adapted to simulate the low frequency electromechanical oscillations over long periods of time (tens of seconds to minutes). In the phasor simulation method, the sinusoidal voltages and currents are replaced by phasor quantities (complex numbers) at the system nominal frequency (50 Hz or 60 Hz).This is the same technique which is used in transient stability softwares. 2. Circuit Description A 9 MW wind farm consisting of six 1.5 MW wind turbines connected to a 25 kV distribution system exports power to a 120 kV grid through a 30 km, 25 kV feeder. A 500 kW resistive load and a 0.9 Mvar (Q=50) filter are connected at the 575 V generation bus. The turbine parameters specifying ratings of power components of the wind turbine are saved in a companion M file (power_wind_dfig_data.m). This file is automatically executed at simulation start so that parameters for the 6x1.5 MW turbine are loaded in your Matlab workspace. (See Model pre-load function specified in the Model Properties). If you want to use this DFIG model with a different rating in another application, you must copy and edit this file in order to change the relevant parameters. Wind turbines using a doubly-fed induction generator (DFIG) consist of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter modeled by voltage sources. The stator winding is connected directly to the 60 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. In this demo the wind speed is maintained constant at 10 m/s. The control system uses a torque controller in order to maintain the speed at 1.09 pu. The reactive power produced by the wind turbine is regulated at 0 Mvar. Double-click on the “Wind Turbine Doubly-Fed Induction Generator (Average Model)” block to see how the model is built. The sample time used to discretize the model (Ts_Power= 50 microseconds) as well as the sample time used by the control system (Ts_Control=100 microseconds) are specified in the Initialization function of the Model Properties. Right-click on the “Control System” block and select “Look Under Mask” to see details of the controller. Open the “Wind Turbine” block menu and check "Display wind-turbine power characteristics". The turbine mechanical power as function of turbine speed is displayed for wind speeds ranging from 6 m/s to 13 m/s. For a wind speed of 10 m/s, the maximum turbine output is 0.55 pu of its rated power (0.55*9MW=4.95 MW) at a speed of 1.09 pu of generator synchronous. 3. Demonstration In this demo you will observe the steady-state operation of the DFIG and its dynamic response to voltage sag resulting from a remote fault on the 120-kV system. Open the “120 kV” block modeling the voltage source and see how a six-cycle 0.2 pu voltage drop is programmed at t=0.03 s Start simulation. Observe voltage and current waveforms on the Scope. At simulation start the “xInitial” variable containing the initial state variables is automatically loaded (from the “power_wind_dfig_avg_xinit.mat” file specified in the Model Properties) so that the simulation starts in steady state. Initially the DFIG wind farm produces 4.8 MW. This active power, corresponds to the maximum mechanical turbine output for a 10m/s wind speed (0.55*9 MW=4.95 MW) minus electrical losses in generator. The corresponding turbine speed is 1.09 pu of generator synchronous speed. The DC voltage is regulated at 1200 V and reactive power is kept at 0 Mvar. At t=0.03 s the positive-sequence voltage suddenly drops to 0.8 p.u. causing an oscillation on the DC bus voltage and on the DFIG output power. During the voltage sag the control system regulates DC voltage and reactive power at their set points (1200 V, 0 Mvar). The system recovers in approximately 4 cycles. Double click the blue block entitled “Show Detailed and Average Simulation Results”. A figure opens showing comparison of the phase A voltage at DFIG terminals, DC link voltage, active and reactive powers and speed for the detailed model and the average model. Notice that the two models are in good agreement. The average model represents correctly the low frequency control and power system

1/9/2004 4:05 AM

Wind Farm (DFIG Average Model)

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oscillations produced by the voltage sag, but voltage waveforms do not show the high frequency harmonics produced by the PWM switching of the two converters. Copyright 1997-2007 The MathWorks, Inc. Published with MATLAB® 7.6 MATLAB and Simulink are registered trademarks of The MathWorks, Inc. Please see www.mathworks.com/trademarks for a list of other trademarks owned by The MathWorks, Inc. Other product or brand names are trademarks or registered trademarks of their respective owners.

1/9/2004 4:05 AM

Wind Farm (DFIG Detailed Model)

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power_wind_dfig_det.mdl

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Wind Farm (DFIG Detailed Model) This demonstration illustrates simulation of a 9 MW wind farm using a detailed model of a Doubly-Fed Induction Generator (DFIG) driven by a wind turbine Richard Gagnon (Hydro-Quebec) Note: This demo uses a generic model of a DFIG wind turbine. The model is useful for education and academic works. Contents 1. Simulation Methods of the DFIG 2. Circuit Description 3. Demonstration

1. Simulation Methods of the DFIG Depending on the range of frequencies to be represented, three simulation methods are currently available in SimPowerSystems™ to model VSC based energy conversion systems connected on power grids. The detailed model (discrete) such as the one presented in this demo. The detailed model includes detailed representation of power electronic IGBT converters. In order to achieve an acceptable accuracy with the 1620 Hz switching frequency used in this demo, the model must be discretized at a relatively small time step (5 microseconds). This model is well suited for observing harmonics and control system dynamic performance over relatively short periods of times (typically hundreds of milliseconds to one second). The average model (discrete) such as the one presentented in the “power_wind_dfig_avg.mdl” model in the DR demo library. In this type of model the IGBT Voltage-sourced converters (VSC) are represented by equivalent voltage sources generating the AC voltage averaged over one cycle of the switching frequency. This model does not represent harmonics, but the dynamics resulting from control system and power system interaction is preserved. This model allows using much larger time steps (typically 50 microseconds), thus allowing simulations of several seconds.

1/9/2004 4:07 AM

Wind Farm (DFIG Detailed Model)

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The phasor model (continuous) such as the one presentented in the “power_wind_dfig” model in the DR demo library. This model is better adapted to simulate the low frequency electromechanical oscillations over long periods of time (tens of seconds to minutes). In the phasor simulation method, the sinusoidal voltages and currents are replaced by phasor quantities (complex numbers) at the system nominal frequency (50 Hz or 60 Hz).This is the same technique which is used in transient stability softwares. 2. Circuit Description A 9 MW wind farm consisting of six 1.5 MW wind turbines connected to a 25 kV distribution system exports power to a 120 kV grid through a 30 km, 25 kV feeder. A 500 kW resistive load and a 0.9 Mvar (Q=50) filter are connected at the 575 V generation bus. The turbine parameters specifying ratings of power components of the wind turbine are saved in a companion M file (power_wind_dfig_data.m). This file is automatically executed at simulation start so that parameters for the 6x1.5 MW turbine are loaded in your Matlab workspace. (See Model pre-load function specified in the Model Properties). If you want to use this DFIG model with a different rating in another application, you must copy and edit this file in order to change the relevant parameters. Wind turbines using a doubly-fed induction generator (DFIG) consist of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The switching frequency is 1620 Hz. The stator winding is connected directly to the 60 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. In this demo the wind speed is maintained constant at 10 m/s. The control system uses a torque controller in order to maintain the speed at 1.09 pu. The reactive power produced by the wind turbine is regulated at 0 Mvar. Double-click on the “Wind Turbine Doubly-Fed Induction Generator (Detailed Model)” block to see how the model is built. The sample time used to discretize the model (Ts_Power= 5 microseconds) as well as the sample time used by the control system (Ts_Control=100 microseconds) are specified in the Initialization function of the Model Properties. Right-click on the “Control System” block and select “Look Under Mask” to see details of the controller. Open the “Wind Turbine” block menu and check "Display wind-turbine power characteristics". The turbine mechanical power as function of turbine speed is displayed for wind speeds ranging from 6 m/s to 13 m/s. For a wind speed of 10 m/s, the maximum turbine output is 0.55 pu of its rated power (0.55*9MW=4.95 MW) at a speed of 1.09 pu of generator synchronous. 3. Demonstration In this demo you will observe the steady-state operation of the DFIG and its dynamic response to voltage sag resulting from a remote fault on the 120-kV system. Open the “120 kV” block modeling the voltage source and see how a six-cycle 0.2 pu voltage drop is programmed at t=0.03 s Start simulation. Observe voltage and current waveforms on the Scope. At simulation start the “xInitial” variable containing the initial state variables is automatically loaded (from the “power_wind_dfig_det_xinit.mat” file specified in the Model Properties) so that the simulation starts in steady state. Initially the DFIG wind farm produces 4.8 MW. This active power, corresponds to the maximum mechanical turbine output for a 10m/s wind speed (0.55*9 MW=4.95 MW) minus electrical and mechanical losses in the generator. The corresponding turbine speed is 1.09 pu of generator synchronous speed. The DC voltage is regulated at 1200 V and reactive power is kept at 0 Mvar. At t=0.03 s the positive-sequence voltage suddenly drops to 0.8 p.u. causing an oscillation on the DC bus voltage and on the DFIG output power. During the voltage sag the control system regulates DC voltage and reactive power at their set points (1200 V, 0 Mvar). The system recovers in approximately 4 cycles. At the end of simulation the THD displayed on the THD_Va_B25 block indicates a 2.2 % of total harmonic distortion on the 25 kV bus B25. You may also display the frequency spectrum of the various voltages and currents stored in the ScopeData structure with time. Use the FFT Analysis tool of the powergui.

1/9/2004 4:07 AM

Wind Farm (DFIG Detailed Model)

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Copyright 1997-2007 The MathWorks, Inc. Published with MATLAB® 7.6 MATLAB and Simulink are registered trademarks of The MathWorks, Inc. Please see www.mathworks.com/trademarks for a list of other trademarks owned by The MathWorks, Inc. Other product or brand names are trademarks or registered trademarks of their respective owners.

1/9/2004 4:07 AM

Wind Farm (DFIG Phasor Model)

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power_wind_dfig.mdl

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Wind Farm (DFIG Phasor Model) This demonstration illustrates phasor simulation of a 9 MW wind farm using Doubly-Fed Induction Generator (DFIG) driven by a wind turbine Richard Gagnon, Bernard Saulnier, Alain Forcione (Hydro-Quebec) Note: This demo uses a generic model of a DFIG wind turbine. The model is useful for education and academic works. Contents Model Description Demonstration

Model Description A 9-MW wind farm consisting of six 1.5 MW wind turbines connected to a 25-kV distribution system exports power to a 120-kV grid through a 30-km, 25-kV feeder. A 2300V, 2-MVA plant consisting of a motor load (1.68 MW induction motor at 0.93 PF) and of a 200-kW resistive load is connected on the same feeder at bus B25. Both the wind turbine and the motor load have a protection system monitoring voltage, current and machine speed. The DC link voltage of the DFIG is also monitored. Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is connected directly to the 60 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. For wind speeds lower than 10 m/s the rotor is running at subsynchronous speed . At high wind speed it is running at hypersynchronous speed. Open the turbine menu, select "Turbine data" and check "Display wind-turbine power characteristics". The turbine mechanical power as function of turbine speed is displayed for wind speeds ranging from 5 m/s to 16.2 m/s. The DFIG is controlled in

1/9/2004 4:04 AM

Wind Farm (DFIG Phasor Model)

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order to follow the red curve. Turbine speed optimization is obtained between point B and point C on this curve. Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generators. The wind-turbine model is a phasor model that allows transient stability type studies with long simulation times. In this demo, the system is observed during 50 s. Open the wind turbine block menu and look at the four sets of parameters specified for the turbine, the generator and the converters (grid-side and rotor-side). The 6-wind-turbine farm is simulated by a single wind-turbine block by multiplying the following three parameters by six, as follows: 1. The nominal wind turbine mechanical output: 6*1.5e6 watts, specified in the Turbine data menu 2. The generator rated power: 6*1.5/0.9 MVA (6*1.5 MW at 0.9 PF), specified in the Generator data menu 3. The nominal DC bus capacitor: 6*10000 microfarads, specified in the Converters data menu Also, notice in the Control parameters menu that the "Mode of operation" is set to " Voltage regulation". The terminal voltage will be controlled to a value imposed by the reference voltage (Vref = 1 pu) and the voltage droop (Xs = 0.02 pu). Demonstration 1. Turbine response to a change in wind speed Open the "Wind Speed" step block specifying the wind speed. Initially, wind speed is set at 8 m/s, then at t = 5s, wind speed increases suddenly at 14 m/s. Start simulation and observe the signals on the "Wind Turbine" scope monitoring the wind turbine voltage, current, generated active and reactive powers, DC bus voltage and turbine speed. At t = 5 s, the generated active power starts increasing smoothly (together with the turbine speed) to reach its rated value of 9 MW in approximately 15 s. Over that time frame the turbine speed will have increased from 0.8 pu to 1.21 pu. Initially, the pitch angle of the turbine blades is zero degree and the turbine operating point follows the red curve of the turbine power characteristics up to point D. Then the pitch angle is increased from 0 deg to 0.76 deg in order to limit the mechanical power. Observe also the voltage and the generated reactive power. The reactive power is controlled to maintain a 1 pu voltage. At nominal power, the wind turbine absorbs 0.68 Mvar (generated Q = -0.68 Mvar) to control voltage at 1pu. If you change the mode of operation to "Var regulation" with the "Generated reactive power Qref " set to zero, you will observe that voltage increases to 1.021 pu when the wind turbine generates its nominal power at unity power factor. 2. Simulation of a voltage sag on the 120-kV system You will now observe the impact of a voltage sag resulting from a remote fault on the 120-kV system. First, in the wind speed step block, disable the wind speed step by changing the Final value from 14 to 8 m/s. Then open the 120-kV voltage source menu. In the parameter "Time variation of", select " Amplitude". A 0.15 pu voltage drop lasting 0.5 s is programmed to occur at t = 5 s. Make sure that the control mode is still in Var regulation with Qref = 0. Start simulation and open the "Grid" scope. Observe the plant voltage and current as well as the motor speed. Note that the wind farm produces 1.87 MW. At t = 5 s, the voltage falls below 0.9 pu and at t = 5.22 s, the protection system trips the plant because an undervoltage lasting more than 0.2 s has been detected (look at the protection settings and status in the "Plant" subsystem). The plant current falls to zero and motor speed decreases gradually, while the wind farm continues generating at a power level of 1.87 MW. After the plant has tripped, 1.25 MW of power (P_B25 measured at bus B25) is exported to the grid. Now, change the wind turbine control mode to "Voltage regulation" and repeat the test. You will notice that the plant does not trip anymore. This is because the voltage support provided by the 5 Mvar reactive power generated by the wind-turbines during the voltage sag keeps the plant voltage above the 0.9 pu protection threshold. The plant voltage during the voltage sag is now 0.93 pu. 3. Simulation of a fault on the 25-kV system Finally, you will now observe impact of a single phase-to-ground fault occurring on the 25-kV line at B25 bus. First disable the 120-kV voltage step. Now open the "Fault" block menu and select "Phase A

1/9/2004 4:04 AM

Wind Farm (DFIG Phasor Model)

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Fault". Check that the fault is programmed to apply a 9-cycle single-phase to ground fault at t = 5 s. You should observe that when the wind turbine is in "Voltage regulation" mode, the positive-sequence voltage at wind-turbine terminals (V1_B575) drops to 0.8 pu during the fault, which is above the undervoltage protection threshold (0.75 pu for a t > 0.1 s). The wind farm therefore stays in service. However, if the "Var regulation" mode is used with Qref = 0, the voltage drops under 0.7 pu and the undervoltage protection trips the wind farm. We can now observe that the turbine speed increases. At t= 40 s the pitch angle starts to increase in order to limit the speed. Copyright 1997-2006 The MathWorks, Inc. Published with MATLAB® 7.6 MATLAB and Simulink are registered trademarks of The MathWorks, Inc. Please see www.mathworks.com/trademarks for a list of other trademarks owned by The MathWorks, Inc. Other product or brand names are trademarks or registered trademarks of their respective owners.

1/9/2004 4:04 AM

Wind Farm (IG)

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power_wind_ig.mdl

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Wind Farm (IG) This demonstration illustrates phasor simulation of a 9-MW wind farm using Induction Generators (IG) driven by variable-pitch wind turbines Richard Gagnon (Hydro-Quebec) Contents Model Description Demonstration

Model Description A wind farm consisting of six 1.5-MW wind turbines is connected to a 25-kV distribution system exports power to a 120-kV grid through a 25-km 25-kV feeder. The 9-MW wind farm is simulated by three pairs of 1.5 MW wind-turbines. Wind turbines use squirrel-cage induction generators (IG). The stator winding is connected directly to the 60 Hz grid and the rotor is driven by a variable-pitch wind turbine. The pitch angle is controlled in order to limit the generator output power at its nominal value for winds exceeding the nominal speed (9 m/s). In order to generate power the IG speed must be slightly above the synchronous speed. Speed varies approximately between 1 pu at no load and 1.005 pu at full load. Each wind turbine has a protection system monitoring voltage, current and machine speed. Reactive power absorbed by the IGs is partly compensated by capacitor banks connected at each wind turbine low voltage bus (400 kvar for each pair of 1.5 MW turbine). The rest of reactive power required to maintain the 25-kV voltage at bus B25 close to 1 pu is provided by a 3-Mvar STATCOM with a 3% droop setting. Open the "Wind Farm" block and look at "Wind Turbine 1". Open the turbine menu and look at the two

1/9/2004 4:03 AM

Wind Farm (IG)

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sets of parameters specified for the turbine and the generator. Each wind turbine block represents two 1.5 MW turbines. Open the turbine menu, select "Turbine data" and check "Display wind-turbine power characteristics". The turbine mechanical power as function of turbine speed is displayed for wind speeds ranging from 4 m/s to 10 m/s. The nominal wind speed yielding the nominal mechanical power (1pu=3 MW) is 9 m/s. The wind turbine model (from the DR library) and the statcom model (from the FACTS library) are phasor models that allow transient stability type studies with long simulation times. In this demo, the system is observed during 20 s. The wind speed applied to each turbine is controlled by the "Wind 1" to "Wind 3" blocks . Initially, wind speed is set at 8 m/s, then starting at t=2s for "Wind turbine 1", wind speed is rammed to 11 m/s in 3 seconds. The same gust of wind is applied to Turbine 2 and Turbine 3, respectively with 2 seconds and 4 seconds delays. Then, at t=15 s a temporary fault is applied at the low voltage terminals (575 V) of "Wind Turbine 2". Demonstration Turbine response to a change in wind speed Start simulation and observe the signals on the "Wind Turbines" scope monitoring active and reactive power, generator speed, wind speed and pitch angle for each turbine. For each pair of turbine the generated active power starts increasing smoothly (together with the wind speed) to reach its rated value of 3 MW in approximately 8s. Over that time frame the turbine speed will have increased from 1.0028 pu to 1.0047 pu. Initially, the pitch angle of the turbine blades is zero degree. When the output power exceed 3 MW, the pitch angle is increased from 0 deg to 8 deg in order to bring output power back to its nominal value. Observe that the absorbed reactive power increases as the generated active power increases. At nominal power, each pair of wind turbine absorbs 1.47 Mvar. For a 11m/s wind speed, the total exported power measured at the B25 bus is 9 MW and the statcom maintains voltage at 0.984 pu by generating 1.62 Mvar (see "B25 Bus" and "Statcom" scopes). Operation of protection system At t=15 s, a phase to phase fault is applied at wind turbine 2 terminals, causing the turbine to trip at t=15.11 s. If you look inside the "Wind Turbine Protections" block you will see that the trip has been initiated by the AC Undervoltage protection. After turbine 2 has tripped, turbines 1 and 3 continue to generate 3 MW each. Impact of STATCOM You will now observe the impact of the "STATCOM". First, open the "Fault" block menu and disable the phase to phase fault. Then put the "STATCOM" out of service by double clicking the "Manual Switch" block connected to the "Trip" input of the "STATCOM". Restart simulation. Observe on " B25 Bus" scope that because of the lack of reactive power support, the voltage at bus "B25" now drops to 0.91pu. This low voltage condition results in an overload of the IG of "Wind Turbine 1". "Wind Turbine 1" is tripped at t=13.43 s. If you look inside the "Wind Turbine Protections" block you will see that the trip has been initiated by the AC Overcurrent protection. Copyright 1997-2006 The MathWorks, Inc. Published with MATLAB® 7.6 MATLAB and Simulink are registered trademarks of The MathWorks, Inc. Please see www.mathworks.com/trademarks for a list of other trademarks owned by The MathWorks, Inc. Other product or brand names are trademarks or registered trademarks of their respective owners.

1/9/2004 4:03 AM